Flyback converters are known in the art and are used in both AC/DC and DC/DC conversion. Flyback converters typically have a buck-boost converter with the inductor split to form a transformer so that the voltage ratios are multiplied with an additional advantage of isolation. When a switch closes, the primary of the transformer directly connects to an input voltage source. The primary current and magnetic flux in the transformer increases, storing energy in the transformer. The voltage induced in the secondary winding is negative, so a corresponding diode is reverse-biased (and hence is blocked). An output capacitor then supplies energy to the output load. When that switch opens the primary current and magnetic flux drops. The secondary voltage is positive and forward-biases the diode, allowing current to flow from the transformer. The energy from the transformer core recharges the capacitor and supplies the load.
Government, industry, and user concerns emphasize high energy efficiency for devices such as AC/DC converters. Some flyback converters employ active clamped flyback to attempt to meet such requirements. Such an approach can achieve some improved efficiency by eliminating switching losses on its switching devices with zero voltage switching (ZVS) capability. Unfortunately, efficiency can still drop off significantly when the converter becomes more lightly loaded, since additional reactive energy from the transformer is needed to perform ZVS (and especially when loading drops to less than fifty percent).
A so-called burst mode of operation can help to improve efficiency for a lightly-loaded flyback converter that employs active clamped flyback. Unfortunately, such a solution can give rise to other problems. As one example in these regards, burst mode control can introduce higher output ripples that in turn require using an oversized output capacitor. As another example in these regards, burst mode control can introduce lower burst frequencies that themselves introduce relatively high audible noise.
Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present teachings. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present teachings. Certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. The terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.